1. Technical Field
The present disclosure relate to an image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof to perform duplex printing.
2. Description of the Related Art
Various image forming apparatuses using electrophotographic technology, in which a toner image is formed on a photoconductor, serving as an image carrier, are widely known. The photoconductor contacts a transfer roller, serving as a nip forming member, to form a transfer nip. By applying a transfer bias to the transfer roller, an electric potential difference that electrostatically moves the toner image on the photoconductor from the photoconductor side to the transfer roller side in the transfer nip is formed in the transfer nip between the photoconductor and the transfer roller.
In single-side printing mode, in which the toner image is formed on only one side of a recording sheet, initially, the recording sheet is sent to the transfer nip in a posture in which the first side of the recording sheet fed from a sheet-feed cassette closely contacts the photoconductor. Then, in the transfer nip, after the toner image is transferred onto the first side of the recording sheet, the recording sheet is transported to the fixing device where the toner image is fixed on the first side of a recording sheet. The recording sheet after passing through the fixing device is then discharged outside of the image forming apparatus.
By contrast, in duplex printing mode, the recording sheet after passing through the fixing device is turned upside down and is transported again to the transfer nip. Then, after the toner image is transferred onto a second side of the recording sheet, the toner image is fixed on the second side of the recording sheet in the fixing device.
In an image forming apparatus disclosed in US Pub 2008/0260401-A, in duplex printing mode, different transfer biases are used when the toner image is transferred onto the first side of the recording sheet and when the toner image is transferred onto the second side thereof. More specifically, when the toner image is transferred onto the first side, a direct-current (DC) bias constituted by only a direct current voltage is applied to the transfer roller as the transfer bias. By contrast, when the toner image is transferred onto the second side, a superimposed bias in which an alternating-current (AC) component is superimposed on a direct-current component is applied as the transfer bias.
With this configuration, deterioration of the photoconductor and the transfer roller caused by electrical discharge can be retarded by using only the DC bias as the transfer bias when the toner image is transferred onto the first side. This is because not a little electrical discharge is generated between the photoconductor and the transfer roller, which accelerates deterioration of the photoconductor and the transfer roller. When the superimposed bias containing the AC component is used as the transfer bias, compared to a case in which the DC bias is used, greater discharge is generated, which hastens the deterioration of the photoconductor and the transfer roller. Alternatively, by using only the DC bias as the transfer bias when the toner image is transferred onto the first side, the deterioration of the photoconductor and the transfer roller can be retarded, compared to the superimposed bias.
It is known that, by using the superimposed bias as the transfer bias instead of the DC bias when the toner image is transferred onto the second side, occurrence of image failure, such as white dots (white voids), toner scattering(fogging), and insufficient image density can be reduced. White dots are phenomena in which toner is partly absent in an image on which the toner should be transferred and the partly absent portion where the color comes out appears as white dots. Toner scattering is a phenomenon in which the toner is scattered and adhered around edges of the image.
The particular mechanism by which this reduction in image failure is accomplished is as follows: When the toner image is transferred onto the second side of the recording sheet, because the recording sheet passes through the fixing device in advance, water evaporates from the recording sheet due to heating in the fixing device. Compared to a state in which the toner image is transferred onto the first side of the recording sheet, the electrical resistance of the recording sheet is increased on transferring the toner image on the first side. Thus, as a transfer current flowing through the image of the recording sheet is decreased, a force to retain the toner in the image weakens, and as a result, the toner scattering and the insufficient image density may be easily generated.
If the value of the DC bias is set greater when the toner image is transferred onto the first side of the recording sheet in order to prevent the occurrence of toner scattering and insufficient image density, the white dots are easily generated, because, as the value of the DC bias is increased, the discharge becomes easily generated between the photoconductor and the transfer roller and facilitates movement (returning movement) of the toner reversely-charged by the discharge in the transfer nip from the image of the recording sheet to the photoconductor. It can be seen that there is thus an inverse relation (trade-off) between generation of the white dots, on the one hand, and toner scattering and insufficient image density on the other.
With the superimposed bias used instead of the DC bias, the trade-off remains but is attenuated because, while the AC electrical field formed between the photoconductor and the transfer roller causes the toner to reciprocally move back and forth between the photoconductor and the recording sheet in the transfer nip, the toner is relatively moved to a surface of the recording sheet. In this process, after the toner scattered around the image of the recording sheet is returned to the surface of the photoconductor, the toner is moved to the image, and as a result, the toner scattering around the image is less likely generated. In addition, when the toner adhering to the surface of the recording sheet is returned to the surface of the photoconductor, the returning toner hits the toner already present on the surface of the photoconductor and promotes separation of the toner from the surface of the photoconductor, which in turn promotes the movement of the toner from the surface of the photoconductor back to the surface of the recording sheet. As a result, the generation of the toner scattering and the insufficient image density are prevented using a smaller DC component, and the white dots are less likely to appear. Accordingly, the toner scattering, the insufficient image density, and the generation of white dots are prevented when the superimposed bias is used as the transfer bias, compared to when the DC bias is used.
However, even though the superimposed bias is used, depending on the exact value of the DC component, the generation of the white dots cannot be prevented sufficiently or the toner scattering and the insufficient image density cannot be prevented sufficiently. Therefore, the image forming apparatus in this example cannot reliably prevent the generation of the white dots, the toner scattering, and insufficient image density.
It is to be noted that, although the problem caused in a configuration in which the toner image is transferred on the recording sheet in the transfer nip formed between the photoconductor and the transfer roller is described above, alternatively, similar problem occur in the following configuration in the transfer nip formed between an image carrier, such as an intermediate transfer member that is different from the photoconductor, and a nip forming member (secondary transfer nip) that contacts the image carrier, and the toner image on the image carrier is transferred on the recording sheet.